The Vibroseis method was initially implemented with a swinging-weight vibrator (Crawford, Doty and Lee; Geophysics v.25 p. 95, 1960). Very soon this was abandoned for hydraulic vibrators. Several hydraulic units could be used in unison and their characteristics were better suited to the recording and correlating technology of the day.
Since then the hydraulic vibrator has become very complex. It is fairly reliable, but very expensive. Further, its limitations of bandwidth have become onerous, as demands increase for better geological resolution. The limitation at the high-frequency end is because the vibrator is basically a constant-force device, because of the compressibility of the hydraulic oil, because of limitations in the servo-valve, and because of the heavy baseplate. These limitations can be offset by sweeping for longer times at the higher frequencies, but this proves to be very expensive. The limitation at the low-frequency end is because of stroke constraints, and because of serious even-order distortion. Again these limitations can be offset by sweeping for longer times at the low frequencies; this is less expensive than at the high frequencies. Particularly at the low frequencies, major phase shifts occur between desired and obtained signals; these must be eliminated by an expensive phase-compensation system.
In contradistinction, the swinging-weight vibrator is essentially simple and inexpensive. Since it is a constant-displacement device, its useful seismic output tends to rise with frequency. Its baseplate can be made light. Its output at low frequency contains little even-order distortion, and although this output is small it can be supplemented by simple changes to the swinging weight.